This invention relates generally to battery-type contacts and interconnect probes and, in particular, to spring-loaded contact probes and a method for biasing the probes which are used in electrical testing applications and battery contact applications.
Conventional spring-loaded electrical contact probes generally include an outer receptacle, a barrel containing a movable plunger and a spring, which exerts a force against the back of the plunger to bias the plunger outwardly against the barrel. The plunger may be depressed inwardly of the barrel a predetermined distance, under force directed against the spring.
Battery-type contacts and interconnect probe designs generally require compact, durable, highly reliable designs with circuit paths optimized for the best performance. These contacts are typically employed in battery charging applications, mobile telecommunication applications, docking applications, and other portable electronic devices in addition to applications for testing electronics, printed circuit boards and computer chips, for example. They may be used as either power conductors or as signal carriers and would be subject to a variety of environmental conditions.
As products continue to shrink in size or increase in performance while maintaining current size, the need for smaller contacts continues to grow. Compliancy of a probe contact though continues to be important to accommodate the tolerances of many parts in an assembly. Many times this compliancy requires a probe with a plunger travel much longer than a spring can supply in the spaced allotted. This is compensated by back drilling the plunger to supply additional space for the spring. The resultant probe performs well mechanically but the electrical performance in certain instances is compromised by the action of the spring and device under test. Specifically, if the device under test pushes directly down on top of the plunger and the spring generates a force pushing directly up the desired contact between plunger and barrel, which is required for optimal electrical performance, can be very light or nonexistent. The result is a poor, unreliable electrical performance for the probe.
As is known in the art, current travels in parallel down all available paths in a quantity dependent upon the path""s resistance. A spring, by nature of its design, has a very large resistance and will cause poor performance if it is the main circuit path. Likewise, large resistances between the barrel inner diameter (xe2x80x9cIDxe2x80x9d) and plunger, referred to as the contact resistance, will also lead to poor performance or failure. Large contact resistances are generally due to low contact force between barrel ID and plunger, poor conductive material of barrel and plunger including plating material and contaminates such as dirt, lint, or even some lubricants. Good probe designs minimize the contact resistance by proper material selection, plating selection, attention to cleanliness/handling, and increasing the contact force between barrel ID and plunger through efforts called biasing, which is the action of forcing the plunger""s bearing surface against the barrel ID.
In an effort to improve biasing in probes many designs have been generated. The most popular and successful has been applying a xe2x80x9cbias cutxe2x80x9d on the tail of the plunger. A large side force is created from the spring pushing against the bias cut creating firm, constant contact force between barrel and plunger. This contact force ensures that the current will flow from the plunger to the barrel and not through the spring and also provides the lowest contact resistance between barrel and plunger. The disadvantage to this type of design is the higher friction that is created between plunger and barrel resulting in failure of the probe due to mechanical wear.
With a back-drilled plunger, an angled surface cannot be generated to induce this biasing. Thus, other techniques must be employed to generate the biasing. Some techniques involve changing the plunger design on the front end to promote biasing while others require special barrels, tangs and such.
The present invention is a plunger with back-drilled hole or aperture with the centerline of the aperture separate from the plunger""s longitudinal axis. The spring force against the plunger is no longer directly in line with the plunger longitudinal axis or centerline. When the plunger encounters the device under test or battery contact, for example, an immediate coupling or moment is created which transfers a portion of the longitudinal force exerted along the plunger axis into a side force. This moment creates the biasing needed by forcing the plunger""s bearing surface against the barrel inner diameter. The pivot point is the contact point between plunger and device under test. The larger the spring force, the larger the moment and thus, the higher the contact force.
Some spring movement or xe2x80x9csnakingxe2x80x9d occurs due to the off-centered hole. The ends of the spring will tend to center themselves in the cavities made for them. Being that the two cavities are not aligned, the spring has no choice but to bend in the center of the coils. This bending action further amplifies the biasing of the plunger if the plunger cavity extends to the center of the spring.
An additional advantage of this design is that the force between plunger and barrel will not be as large as a normal biased plunger and will result in longer life through less wear. In a normal biased design the wear is localized between the plunger and barrel due to the severe biasing of the plunger. This new design with a less aggressive biasing, spreads the wear more evenly across the contact points of the barrel and plunger thereby reducing wear and increasing the life of the probe. Additionally, a slight random rotation of the plunger in the barrel due to the spring action further spreads and reduces the wear.